The use of switched mode output stage(s) in audio amplification, in which the conversion is digitally performed, allows audio output from digital signal input. The digital signal conversion may be achieved by a variety of devices. One particular technique utilizes a delta-sigma (ΔΣ) modulator, in which the digital input signal is filtered and then quantized to produce an audio output. In one switched technique, the noise-shaped quantized signal from the quantizer is coupled to a pulsewidth modulator (PWM), in which a value of the quantized signal controls the duty cycle of the PWM output. The PWM output then controls the operation of a switching device utilized as the output stage of an amplifier to drive a load. Thus, the filtered digital signal is utilized to modulate the pulsewidth of the PWM signal to control the duration of the switch on/off time of the output drive stage to the load.
For PWM driven outputs, the output may need to be switched off when certain situations occur. Typically, an error in the output circuit may indicate the potential for harm to the circuit. For example, the error may be in the form of operating the output device outside their safe operating limits with respect to a certain circuit parameter(s), such as excessive voltage, current, and/or temperature. A variety of circuits may be implemented to monitor and detect the various error conditions that may affect the performance of the output stage of an amplifier, including conditions that may pose potential damage to the amplifier. These detection circuits may then generate a shutdown signal to shutdown the output drive if an undesirable condition exists and/or persists.
However, when the shutdown occurs asynchronously to the timing of the PWM pulse, a potential danger exists for generating shutdown pulses shorter than the minimum width required to control the output devices. This situation is due to some amount of minimum pulse duration generally needed for properly turning on/off of the output driver transistors. If the shutdown is activated immediately after the rising edge or if the shutdown is released immediately before the falling edge of the PWM signal controlling the output stage, the shutdown control signal may result in the generation of an improper PWM signal to drive the load. An improper PWM signal of insufficient duration may not drive the output stage correctly and/or may place the output stage in some other state than the desired state. Accordingly, when shutdown of PWM circuitry is dependent on an asynchronous signal, some form of timing control may be needed to ensure that PWM signals of less than minimum selected pulse duration are not generated to control the output drive stage of an amplifier, such as an audio amplifier.